While semiconductor-on-insulator (SOI) field effect transistors offer faster operational speed than a bulk field effect transistor of comparable dimensions due to the reduction in capacitive coupling between the transistor and the underlying semiconductor substrate, SOI field effect transistors suffer from some adverse effects due to the presence of a floating body in the device structure. One manifestation of such adverse effects is an increased susceptibility to bipolar breakdown at a lower voltage compared with a bulk field effect transistor having similar dimensions.
Referring to FIG. 1, the mechanism of such an increase in the susceptibility to bipolar breakdown is illustrated for an exemplary prior art semiconductor-on-insulator (SOI) field effect transistor (FET). The exemplary prior art SOI FET comprises a semiconductor-on-insulator (SOI) substrate 108, a gate dielectric 140, and a gate electrode 150. The SOI substrate 108 comprises a handle substrate 110, a buried insulator layer 120, and a top semiconductor layer 130. The top semiconductor layer 130 comprises a body region 132, a source region 134, and a drain region 136. The body region 132 has a doping of a first conductivity type, which may be p-type or n-type, and the source region 134 and the drain region 136 have a doping of a second conductivity type, which is the opposite type of the first conductivity type. A depletion region, represented by the area between two dotted lines in the top semiconductor layer 130, is formed around the interface between the body region 132 and the drain region 136.
During the operation of the exemplar prior art SOI FET, a high drain electric field causes impact ionization in the depletion zone. At least one electron-hole pair is generated by the impact ionization, and the electron(s) and the hole(s) are attracted in opposite directions depending on the doping of the body region 132 and the drain region 136. For example, if the exemplary prior art SOI FET is an n-type field effect transistor, in which the body region 132 has a p-type doping and the drain region 136 has an n-type doping, holes from the impact ionization diffuse away from the drain region 136 into the body region 132 where the holes can accumulate, and electrons from the impact ionization diffuse away from the body region 132 and into the drain region 136, e.g., to the power supply, as illustrated in FIG. 1. Correspondingly, if the exemplary prior art SOI FET is a p-type field effect transistor, in which the body region 132 has an n-type doping and the drain region 136 has a p-type doping, electrons from the impact ionization diffuse away from the drain region 136 into the body region 132 and holes from the impact ionization diffuse away from the body region 132 and into the drain region 136, e.g., to the power supply.
In the case of an n-type SOI field effect transistor, the holes that accumulate in the body region 132 prior to flowing into the source region 134 raise the potential of the body region 132. As the p-n junction at the interface between the body region 132 and the source region 134 become forward biased, electrons are injected from the source region 134 into the body region 132, and travel to the depletion region at the interface between the body region 132 and the drain region 136. Additional electron-hole pairs are generated by impact ionization caused by these electrons. Additional holes are injected into the body region 132 and additional electrons are injected into the drain region 136. A positive feedback mechanism is set in motion, which results in bipolar breakdown of the exemplary prior art SOI FET. A similar mechanism may also cause bipolar breakdown in a p-type SOI field effect transistor, in which the polarity of charge carriers is reversed, i.e., holes replace electrons.
The response of a drain current (I_drain) as a function of a drain voltage (V_drain) while the exemplary prior art SOI FET is turned on is shown in FIG. 2. The drain current is substantially constant until the drain voltage reaches a certain value. At a critical value of the drain voltage, which is herein termed a “bipolar breakdown voltage” V_bd, the drain current increases abruptly due to the positive feedback mechanism described above. The bipolar breakdown voltage is lower for the exemplary prior art SOI FET compared with a bulk field effect transistor because charge carriers, e.g., holes, that accumulated in the exemplary prior art SOI FET cannot escape to any other node than into the source region 134, and consequently, tend to accumulate in the body region 132, while the charge carriers may flow into other portions of the substrate in the bulk field effect transistor formed in a bulk substrate. In other words, the floating body configuration of the exemplary prior art SOI FET prevents dissipation of the accumulated charge carriers from the initial impact ionization, while a conductive path between a body region and the bulk of the semiconductor substrate provided in a bulk field effect transistor dissipates accumulated charges from the initial impact ionization. Thus, lack of mechanism to dissipate the charge carriers that build up in the body region 132 in the exemplary prior art SOI FET causes reduction of the bipolar breakdown voltage relative to a bulk field effect transistor having similar dimensions.
Such a reduction in the bipolar breakdown voltage places a limitation on the operation of the exemplary prior art SOI FET. Particularly, in applications wherein a high drain voltage is required, the reduced bipolar breakdown voltage renders the exemplary prior art SOI FET severely disadvantaged relative to equivalent bulk field effect transistors. However, formation of bulk and SOI devices on the same substrate typically involves additional processing steps and increased manufacturing cost, let alone increases in the complexity of design and increased design cost.
In view of the above, there exists a need to provide an SOI field effect transistor having a high bipolar breakdown voltage, and methods of manufacturing the same.